When air that has been once inspired is examined, it is found to have undergone considerable alteration. The proportion of nitrogen, indeed, is not materially altered - it still remains at about 79 in 100 parts, - but about 5 parts of oxygen have been abstracted, leaving the percentage of oxygen at about 16 per cent, while the quantity of carbon dioxide is increased from 1 part in 2500 to 112.5 parts in 2500, or to 4.5 per cent. The air is, in addition, rendered warmer, providing, of course, that when it was inspired it was not higher in temperature than that of the body. When this occurs, as it sometimes does in hot climates, the air inspired may actually be co led by passing through the lungs. As a rule, the temperature of the expired air is raised to nearly 98°. Moreover, it is saturated with watery vapour which has evaporated from the moist mucous tracts of the nose, trachea, and alveoli of the lungs. It is also freed from all particles of dust and from all living or dead germs, which adhere to the moist surface of the respiratory tract, to be tossed outwards by the wave-like movements of the cilia of the ciliated cells lining the air-passages, and ultimately to be coughed up. Finally, some volatile gases or oils are added to it, as is proved by the peculiar odour of the breath characteristic of each animal, and especially observable after the use of certain articles of diet, such as garlic.

Putting the changes in the air in a tabular form, they may be thus represented: -

Before inspiration.

After expiration.

Oxygen ......

21

16

Nitrogen ...

79

79

Carbon dioxide

0.04

45

Aqueous vapour ...

1 or more per cent ......

Saturated

Temperature

Variable

About 98°

More or less germ-laden

Germ-free

It has been found that in the case of man, and probably therefore also in the horse, climate materially affects the absorption of oxygen. Thus, under ordinary conditions of respiratory activity in the hot climate of Madras, a man absorbs in one month 177 lbs. of oxygen, in the drier air of London or Brussels, 192 lbs., in the still drier atmosphere of St. Petersburg or Barnaul in Tomsk, 199 lbs., and in the latter place in winter, when the quantity of moisture in the air is at a minimum, as much as 218 lbs. These differences in some measure account for the languor experienced in hot climates, and for the briskness and great heat-producing power exhibited by the body in cold climates.